It's all about the quality of your eggs. Women's egg quality and availability diminishes with age. There are tests your OB-Gyn can run to check those levels to look at your egg reserves. Some wonen experience early menopause. There's no good "one size fits all" number when it comes to what age that hapoens. For some women it may be 25, for others it may be 40. You can always do egg freezing now, to retrieve your eggs and save them for later. Even if you waited to use them for 10 or 15 years, the eggs will be frozen in time and not age. Women can have healthy, successful pregnancies into their mid to late 40s, (I've even seen some women become pregnant at 52!) but your eggs won't stay good that long. Here's a medically reviewed article you may find helpful. https://www.fertilitywise.com/research/egg-freezing

Here's a good medically reviewed article that might have some good insight for you. Overall, there's a lot of things people think impact fertility but ultimately don't really have any true weight behind them. Being healthy and making healthy decisions is the best thing you can do! https://www.fertilitywise.com/research/getting-pregnant-via-sex

I really like this article (and heck-the whole site because it's medically reviewed, not just someone's biased opinion, and the articles tend to be easier to read than ASRM or other medical articles) and thought you might find it helpful. Even though it's meant for getting pregnant via sex, it's got some good points about "how to optimize fertility" that you might find helpful! https://www.fertilitywise.com/research/getting-pregnant-via-sex

You read my mind! It was the same for me. They were monitoring and were ready to freeze embryos and switch to an FET instead of fresh transfer, but I was already there... I know sometimes implantation can trigger symptoms to get worse, so maybe that's what he's implying??

Here is a medically reviewed article that you could share with your sister. Also, a lot of people with genetic concerns opt to use donor eggs or donor embryos, so they don't have to worry about passing that genetic concern on. But this article is a good place to start! https://www.fertilitywise.com/research/preimplantation-genetic-testing-pgt